An R—Fe—B based rare-earth sintered magnet, including an Nd2Fe14B type compound phase as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances. It is already known, however, that an R—Fe—B based rare-earth sintered magnet will cause an “irreversible flux loss” (i.e., a phenomenon that a magnet will lose more and more magnetism thereof as the temperature rises). For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss. To realize that, the coercivity of the magnet at an ordinary temperature needs to be increased or the absolute value of the rate of variation in coercivity to a required temperature (i.e., the temperature coefficient of the coercivity) needs to be decreased.
It has been known that if the rare-earth element R in the R2Fe14B phase is replaced with a heavy rare-earth element RH (which may be Dy and/or Tb), the coercivity will increase. In that case, the temperature coefficient of the coercivity also increases proportionally to the percentage of the rare-earth element R replaced by the heavy rare-earth element RH. That is why it has been believed to be effective to add such a heavy rare-earth element RH as much as possible to achieve high coercivity at a high temperature.
Among other things, since the magnetocrystalline anisotropy of Tb2Fe14B is approximately 1.5 (= 3/2) times as high as that of Dy2Fe14B, the coercivity and the temperature coefficient of the coercivity can be increased more efficiently with Tb than with Dy.
However, the magnetic moments of the heavy rare-earth element RH in the R2Fe14B phase and Fe have mutually opposite directions. That is why the greater the percentage of the light rare-earth element RL (which may be at least one of Nd and Pr) replaced with the heavy rare-earth element RH, the lower the remanence Br would be. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use is preferably cut down. For these reasons, the coercivity of a rare-earth magnet should be increased effectively with the addition of as small an amount of the heavy rare-earth element RH as possible.
Patent Document No. 1 discloses that by adjusting the ratios of the light and heavy rare-earth elements RL and RH and the mole fraction of another constituent element of an R—Fe—B based rare-earth magnet within predetermined ranges, the temperature coefficient of the R—Fe—B based rare-earth magnet will increase.
Patent Document No. 2 teaches raising the temperature, at which the percentage of the irreversible flux loss of an R—Fe—B based rare-earth magnet reaches 5%, by 30° C. or more compared to the conventional technique by carrying out aging treatment in two stages after the sintering process.
Patent Document No. 3 discloses that by making an R—Fe—B based rare-earth magnet of a mixture of a hard magnetic material power, including a rare-earth element, and a diamagnetic material powder, magnetic coupling will be produced between the hard magnetic material powder and the diamagnetic material power, thus reducing the absolute value of the temperature coefficient of the R—Fe—B based rare-earth magnet.
Patent Document No. 4 teaches how to increase the magnetic transformation temperature and the temperature coefficient by adding a ferromagnetic fluorine compound to an R—Fe—B based rare-earth magnet.
Patent Document No. 5 discloses that if a rare-earth-iron-boron based magnet is held in a pressure-reduced chamber so that an element M (which is one, two or more rare-earth elements selected from the group consisting of Pr, Dy, Tb and Ho), which has turned into vapor or fine particle by some physical technique, or an alloy including such an element M, is deposited to form a film on the surface of a magnet and then caused to diffuse and permeate, a crystal grain boundary layer, including plenty of the element M, is formed. In that case, even if the concentration of the rare-earth element such as Dy were reduced, a high-performance magnet with high coercivity or high remanence could still be obtained according to Patent Document No. 5.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-284111    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 5-47533    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2004-79922    Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2005-209669    Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2005-11973